The present invention relates to a method and apparatus applied in an optical disc drive for obtaining a push-pull signal, and more particularly, to a method and apparatus applied in an optical disc drive for obtaining a push-pull signal via adjusting amplifying gains according to fluctuating signals generated from a photo detector of a pick-up head.
As well known to those skilled in the art, an optical disc drive reads information recorded on a wobble track spirally formed on an optical disc to identify physical addresses used for accessing the optical disc or to extract a reference clock (i.e., the wobble clock) used for generating clock signals required by the optical disc drive. The wobble track defines a wavy wobble groove on the optical disc. Therefore, when the pick-up head moves along the track, the pick-up head outputs a laser beam onto the optical disc and then detects intensity of the laser beam reflected from the optical disc. Through comparing intensity of the reflected laser beam detected by two separate sides of a photo detector of the pick-up head, a signal commonly known as a “push-pull signal” is obtained. The push-pull signal is a periodic sinusoidal signal whose period is related to the position of the pick-up head and the rotating speed of the optical disc, so it is suitable to be referenced by the optical disc drive. In short, it is very important to extract an accurate, undistorted push-pull signal so as to achieve high stability and good performance.
As mentioned above, the push-pull signal is applied to identify physical addresses on the optical disc. Taking a DVD-R/RW disc for example, there are some land pre-pits pre-recorded on the wobble track, resulting in spike patterns included in the generated push-pull signal. Please refer to FIG. 1 in conjunction with FIG. 2. FIG. 1 is a diagram of a pick-up head 10 moving on an optical disc 50 according to the related art. FIG. 2 is a diagram of a push-pull signal S1 generated according to signals detected by a photo detector 12 of the pick-up head 10 shown in FIG. 1. As shown in FIG. 1, a small area on an optical disc 50 is enlarged, showing a plurality of wobble track segments 30a, 30b and a plurality of data track segments 32a, 32b. These wobble track segments 30a, 30b belong to a single wobble track spirally formed on the optical disc 50, and these data track segments 32a, 32b belong to a single data track spirally formed on the optical disc 50. The wobble track segments 30a and 30b are land regions, while data track segments 32a and 32b are groove regions. In addition, as shown in FIG. 1, there are a plurality of data patterns 34 recorded on the data track segments 32a and 32b. 
On the optical disk 50, due to intensity of the reflected laser beam in the land region being different from the groove region, the four areas A, B, C, and D of a photo detector 12 of the pick-up head 10 detect different intensity of the reflected laser beam when the pick-up head 10 moves along wobble track segment 32a according to the direction T. Generally, the intensity of the laser beam detected by areas B and C is subtracted from the intensity of the laser beam detected by areas A and D to generate a push-pull signal S1. Please note that a land pre-pit 20 disrupts wobble track segment 30b as shown in FIG. 1, and when the pick-up head 10 moves across the land pre-pit 20, the corresponding push-pull signal S1 shown in FIG. 2 has a spike due to asymmetry between two sides of the wobble track segments 30a, 30b. Therefore, if a spike is detected, the optical disc drive knows that a land pre-pit 20 is located at the current position of the pick-up head 10. User data are recorded onto the optical disc 50 according to the corresponding address, and the land pre-pits materialize the addressing on the optical disc 50. Therefore, how to obtain a correct push-pull signal S1 with an easy-detecting spike relates to performance of the optical disc drive.
Please refer to FIG. 3. FIG. 3 is a diagram of a conventional apparatus for generating a push-pull signal S1. The pick-up head 10 detects a reflected laser beam from an optical disc by a photo detector 12, and four areas A, B, C, and D of a photo detector 12 output four detecting signals IA, IB, IC, and ID, respectively. Next, these detecting signals IA, IB, IC, and ID pass through pre-amplifiers 100a-100d respectively, becoming amplified detecting signals IA′, IB′, IC′, and ID′. Then, the amplified detecting signals IA′, IB′, IC′, and ID′ are respectively transferred into following gain adjusters 110a-110d. The purpose of the gain adjusters 110a-110d is for further enlarging amplitudes of the amplified detecting signals IA′, IB′, IC′, and ID′ to obtain an easily distinguished push-pull signal S1. Amplified signals IB″ and IC″ are summed up through an adder 130a and then transferred into a balance adjuster 120a for eliminating asymmetry, and amplified signals IA″ and ID″ are summed up through an adder 130b and then transferred into a balance adjuster 120b. Finally, a subtractor 140 subtracts these two balanced signals outputted from the balance adjusters 120a and 120b to generate the wanted push-pull signal S1. As described above, one of the key methods of getting the push-pull signal S1 is gain adjusting performed by the gain adjusters 110a-110d to maximize the amplitudes of detecting signals IA-ID.
In the normal operations of optical disc drives, there are three basic modes—read mode, write mode, and blank mode. When the optical disc drive is reading data from or writing data into the optical disc, the pick-up head 10 detects a reflected laser beam containing data information and wobble information such that the detecting signals IA-ID are mixed with pulse signals. Unfortunately, the amplitude of the pulse signals (i.e. write pulses in write mode or pulses caused by EFM signal recorded in the groove in read mode) is much larger than the amplitude of wobble signal; therefore each gain adjuster 110a, 110b, 110c, 110d determines the voltage gain mostly according to the amplitude of pulse signals, resulting in a small voltage gain applied to the incoming wobble signal. When the optical disc drive is in the blank mode, there is no pulse signal to interfere with the wobble signal, but because the gain of each gain adjuster 110a, 110b, 110c, 110d is fixed according to the amplitude of pulse signals in read or write mode, the optical disc drive is sure to have bad performance owing to the small gain.